In the conventional chain drive used to transmit rotation of a crank shaft of an engine to a cam shaft, as shown in FIG. 6, a chain 17 is passed over between a drive sprocket 12 secured to a crank shaft 11 and driven sprockets 15, 16 secured to cam shafts 13, 14, respectively. A tensioner lever 20 is supported pivotally around a support shaft 18 secured on the engine side and biased toward the chain 17 by means of a tensioner 19. The tensioner lever 20 is arranged along the outer periphery of the chain 17 between the drive sprocket 12 and the driven sprocket 15. The chain 17 is pressed from the side by a shoe 21 of the tensioner lever 20 to prevent looseness thereof and vibrations during running.
Further, a fixed chain guide 22 for preventing vibrations during running of the chain 17 is arranged along the outer periphery of the chain 17 between the drive sprocket 12 and the driven sprocket 16.
The fixed chain guide 22 is composed of a shoe 23 in sliding contact with the chain 17 and a plate base 24 mounted on the shoe 23, and the plate base 24 is secured to a mounting surface on the engine side by two mounting seats 25, 26 which extend for less than one-fourth of the length of the plate base 24 adjacent opposite ends of the base. Between the seats 25 and 26, the base is bowed outwardly to guide the chain 17 in a longitudinal direction.
In the process of manufacturing the plate base of the chain guide described above, a metal plate stock is externally punched. Punching shears the plate stock to produce a longitudinal edge having a shear surface facing the punch and a rupture surface on the opposite surface. After punching, the mounting seats are subjected to bending approximately perpendicular to the plate base. However, in the case where two mounting seats are bent oppositely to each other with respect to the longitudinal direction of the plate base, as shown in FIG. 6, in both edges of the bent portion of one mounting seat, the rupture surface caused by the external punching is positioned externally in the bending direction. Therefore, cracks sometimes occurred externally in the bending direction, during the bending of the mounting seats, resulting from fine cracks included in the rupture surface.
Further, since the chain guide is incorporated into a narrow space internally of the engine, when a height h from the bottom of the mounting seat 25 (26) to a bottom edge 24B of a shoe support portion 24A of the plate base 24 is low, as shown in FIG. 7, an escape internally in the bending direction disappears when the mounting seats 25, 26 are subjected to bending, posing a problem in that cracks occur in the bend portion and the accuracy of the bend is reduced.
On the other hand, a proposal has been made to solve the above problem by an arrangement wherein for securing the aforementioned escape, a slit-like escape portion G is formed from both end edges widthwise of mounting seat 25' (26') to a shoe support portion 24'A of a plate base 24', as shown in FIG. 8, after which bending is carried out. In this proposal, however, stress tends to be concentrated on the escape portion G entered in the shoe support portion 24' to lower the strength of the plate base 24', posing a practical problem.